Luminescent object and utilization thereof

ABSTRACT

A first aspect of the invention relates to a luminescent object comprising: a. a luminescent layer or core containing a photoluminescent material; and b. a wavelength-selective mirror; wherein the luminescent layer or luminescent core is optically coupled to the wavelength-selective mirror, said wavelength-selective mirror being at least 50% transparent to light absorbed by the photoluminescent material and at least 50% reflective to radiation that is emitted by the photoluminescent material. The luminescent object according to the present invention may advantageously be employed in luminescent solar concentrator systems as it enables highly efficient transportation of radiation emitted by the photoluminescent material following exposure to incident solar light. Another aspect of the invention concerns a photovoltaic device comprising an electromagnetic radiation collection medium containing the aforementioned luminescent object and a photovoltaic cell capable of converting optical radiation to electrical energy which is optically coupled to the luminescent object. Further aspects of the invention include a fluorescent light activated display and a room lighting system comprising the aforementioned luminescent object.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a luminescent object and in particularto the application of such a luminescent object in optical luminescentconcentrator devices, such as luminescent solar concentrator devices.

BACKGROUND OF THE INVENTION

The cost of solar energy per unit Watt is approximately 5-10 timeshigher than energy from other sources, which include coal, oil, wind,biomass and nuclear. In order to reduce the cost of solar energygeneration in photovoltaic systems, it is desirable to make efficientuse of the most expensive part of the system, namely the photovoltaiccell (solar cell). Conventionally, this is done by using largelight-focusing solar concentrators (parabolic or trough dishes). Thesedevices have several disadvantages, including high investment cost, highmaintenance cost, unwieldy shapes, and the necessity of tracking the sunas it crosses the sky: for a review of current state of the art, seeSwanson, Progress in Photovoltaics: Research and Applications 8, 93(2000).

An alternative option that has been the subject of investigations is touse a waveguide that collects the light and transports it to a smallphotovoltaic cell. Some of these efforts attempted to use holographicmeans (U.S. Pat. No. 5,877,874) or geometrical optics to redirect thelight (see for example T. Uematsu et al Sol Energ Mater Sol C 67, 415(2001) and U.S. Pat. No. 4,505,264). These attempts were ratherunsuccessful, especially for large transport distances because theefficiencies were low or the systems require tracking of the sun or thesystems were complex and not suitable for large-scale production orcombinations thereof.

Luminescent solar concentrators (LSC) represent another alternative thathas been the subject of investigations, predominantly because thesesystems are easy to produce at low cost and because these systems do notrequire tracking of the sun. LSCs consist basically of a large glass orpolymeric plate, sheet, film, fibre, ribbon, woven or coating which isdoped with fluorescent dye molecules. The dyes absorb light of specificwavelengths from the solar light incident upon it, and re-emit the lightin all directions at a longer wavelength. A portion of this light isemitted within the critical angle of the supporting waveguide, and istotally internally reflected and transported to the photovoltaic cell.The LSC has the advantage of combining less expensive materials withflexibility (especially when a plastic waveguide is used) without theneed of a heat sink or a sun tracking system. A sample system with adifferent purpose (room lighting) is described in Earp et al, Sol EnergMat Sol C 84, 411 (2004). At the moment, LSC-systems are not usedcommercially which is predominantly related to their poor efficiency.This low overall efficiency originates from a high re-absorption ofemitted light (limited Stokes Shift of the dye), from a poor efficiencyof coupling light into the waveguide and from a poor efficiency inkeeping the light within the waveguide.

The present invention aims to remedy these drawbacks of LSC-systems, inparticular by providing means of increasing the efficiency with whichemitted light is kept within the LSC-system.

SUMMARY OF THE INVENTION

The inventors have discovered that the efficiency of LSC-systems can beincreased substantially by employing (a) a luminescent layer orluminescent core containing a photoluminescent material in combinationwith (b) one or more wavelength-selective mirrors that are largelytransparent to light absorbed by the photoluminescent material and thatstrongly reflect optical radiation that is emitted by thephotoluminescent material. In a preferred embodiment, there is provideda luminescent object comprising a luminescent layer or core containing aphotoluminescent material and a wavelength-selective mirror, wherein theluminescent layer or luminescent core is optically coupled to thewavelength-selective mirror, said wavelength-selective mirror being atleast 50% transparent to light absorbed by the photoluminescent materialand at least 50% reflective to radiation that is emitted by thephotoluminescent material and wherein the wavelength-selective mirrorcomprises a cholesteric layer of chiral nematic polymer.

The aforementioned wavelength-selective mirror can suitably bepositioned as a separate layer anywhere between the luminescentlayer/core and the surface that is meant to receive incident opticalradiation. Thus, incident light will pass through thewavelength-selective mirror to excite the photoluminescent materialcontained in the underlying luminescent layer or luminescent core. Theoptical radiation emitted by the photoluminescent material that hits thewavelength-selective mirror will be reflected, thus preventing saidemitted radiation from escaping the LSC. As a result emitted radiationis concentrated highly effectively within the LSC, resulting in animproved overall efficiency.

The wavelength-selective mirror may also advantageously be applied onthe opposite side of the luminescent layer, i.e. on the side opposite ofthe side receiving incident light. Thus, it may be ensured that emittedradiation is reflected back into the luminescent layer or into awaveguide.

In order to realise the benefits of the present invention thewavelength-selective mirror must be largely transparent to radiationthat is capable of exciting the photoluminescent material and at thesame time said mirror must effectively reflect the radiation emitted bysaid photoluminescent material. Accordingly, the one or morewavelength-selective mirrors employed in the luminescent object of thepresent invention are at least 50% transparent to light absorbed by thephotoluminescent material and at least 50% reflective to radiation thatis emitted by the same photoluminescent material.

Hence, the invention provides a luminescent object comprising aluminescent layer or core containing a photoluminescent material; and awavelength-selective mirror; wherein the luminescent layer orluminescent core is optically coupled to the wavelength-selectivemirror, said wavelength-selective mirror being at least 50% transparentto light absorbed by the photoluminescent material and at least 50%reflective to radiation that is emitted by the photoluminescent materialand wherein the wavelength-selective mirror comprises (a) a polymericstack layer, and/or (b) a cholesteric layer of chiral nematic polymer.

In a specific embodiment, the invention provides luminescent objectcomprising a luminescent layer or core containing a photoluminescentmaterial; and a wavelength-selective mirror; wherein the luminescentlayer or luminescent core is optically coupled to thewavelength-selective mirror, said wavelength-selective mirror being atleast 50% transparent to light absorbed by the photoluminescent materialand at least 50% reflective to radiation that is emitted by thephotoluminescent material and wherein the wavelength-selective mirrorcomprises a cholesteric layer of chiral nematic polymer, preferablycomprising a first cholesteric layer reflecting right-handed circularlypolarized light and a second cholesteric layer reflecting left-handedpolarized light.

In yet another specific embodiment, the invention provides luminescentobject comprising a luminescent layer or core containing aphotoluminescent material; and a wavelength-selective mirror; whereinthe luminescent layer or luminescent core is optically coupled to thewavelength-selective mirror, said wavelength-selective mirror being atleast 50% transparent to light absorbed by the photoluminescent materialand at least 50% reflective to radiation that is emitted by thephotoluminescent material and wherein the wavelength-selective mirrorcomprises a polymeric stack layer, preferably comprising a firstpolymeric stack layer reflecting one polarization of light and a secondpolymeric stack layer reflecting the opposite polarization of light.

Examples of wavelength-selective mirrors that may advantageously beemployed in accordance with the present invention include polymericstacks and cholesteric layers of chiral nematic polymer.

According to an aspect of the invention, the invention relates to aluminescent object comprising an aligned polymer that contains anoriented photoluminescent material, said aligned polymer having apretilt angle of 10-90° relative to the surface of the object. In aspecific embodiment, the invention is directed to a luminescent objectcomprising a luminescent layer and a waveguide, wherein the object is anoptical laminate or an optical fibre, the luminescent object beingcoupled optically to the waveguide, the luminescent object comprising analigned polymer that contains an oriented photoluminescent material,said oriented photoluminescent material being immobilized within thealigned polymer, and said aligned polymer having a pretilt angle of10-89°, preferably 10-90°, more preferably 10-85°, even more preferably15-85°, yet even more preferably 30-80°, more preferably 30-70°, yeteven more preferably 40-70°, relative to the surface of the object.

This luminescent object may be used to convert incident light into lightof a longer wavelength. In case the emitted light is radiated at arelatively small angle relative to the surface of the object (requiringthe use of a relatively high pretilt angle), the emitted light can betransported efficiently within the plane parallel to said surface to,for instance, an exit or a photovoltaic device. Thus, the present filmcan be applied as such, without a separate waveguide, in e.g. LSCs. Inthis particular case it is very advantageous to use photoluminescentmaterials with a large Stokes Shift and/or with little overlap betweenthe absorption and emission spectra to avoid large light losses byre-absorption phenomena.

Definitions

The term “luminescent” as used herein refers to the capability of amaterial to emit light upon absorption of light or other radiation ofsufficient quantum energy. The term includes both fluorescence andphosphorescence.

The term “light” as used herein refers to optical radiation which may bevisible or invisible to the human eye.

The term “optical radiation” refers to electromagnetic radiation in thewavelength range between 100 nm and 2000 nm.

The term “photoluminescence” as used herein refers to luminescencegenerated by the absorption of light.

The term “photoluminescent material” as used herein refers to atoms ormolecules, including ions that are capable of photoluminescence. Theterm “photoluminescent material” also encompasses combinations of two ormore different photoluminescent components, e.g. combinations of two ormore different photoluminescent molecules. The term “photoluminescentmaterial” also encompasses guest-host systems comprising a fluorescentmolecule, fluorescent polymers and/or co-polymers.

The term “reflective” as used herein means that a material reflects mostincident solar light and/or light emitted by the photoluminescentmaterial. More particularly, the term “reflective” means that saidmaterial reflects at least 50%, preferably at least 60%, more preferablyat least 80% and most preferably at least 90% of said light. Thereflectivity of a material is determined for light incidentperpendicular to the reflective surface.

The term “transparent” as used herein means that a material transmitsmost incident solar light and/or light emitted by the photoluminescentmaterial. More particularly, the term “transparent” means that saidmaterial transmits at least 50%, preferably at least 70%, morepreferably at least 90% of said light, measured for light incidentperpendicular to the surface of the object that is exposed to saidincident light.

The term “polymeric stack” refers to multilayer films containingsub-layers with different refractive indices based on organic(polymeric) materials that exhibit wavelength selectivity, optionally incombination with polarization selectivity: see, for instance, U.S. Pat.No. 6,157,490.

The term “wavelength selective mirror” as used herein refers to mirrorswhich are transparent at specific wavelengths and reflective at otherwavelengths, optionally in combination with polarization selectivity. Avariety of such mirrors are known in the literature.

The terminology “cholesteric layer of chiral nematic polymer” refers toa layer comprising polymers whose mesogenic groups are alignedpredominantly parallel to the surface of the layer and in which themolecules rotate with respect to each other in a pre-specified directionwhich is induced by a chiral reactive or non-reactive dopant.Especially, the terminology “cholesteric layer of chiral nematicpolymer” refers to a layer that has a chiral nematic phase that exhibitschirality (handedness). This phase is often called the cholesteric phaseas it was first observed for cholesterol derivatives. Chiral molecules(those molecules that lack inversion symmetry), either reactive ornon-reactive, can give rise to such a phase. This phase exhibits atwisting of the molecules along the director, with the molecular axisperpendicular to the director. The finite twist angle between adjacentmolecules is due to their asymmetric packing, which results inlonger-range chiral order. The chiral pitch refers to the distance(along the director) over which the mesogens undergo a full 360° twist.The pitch may be varied by adjusting temperature or adding othermolecules to the LC fluid.

These wavelength selective mirrors can be wavelength tuned (see forexample Katsis et al (1999) Chem. Mater. 11, 1590)) or bandwidth tuned(see for example Broer et al (1995) Nature 378, 467).

The term “waveguide” as used herein refers to optical components thatare transparent to light and that confine optical radiation from aninput to a desired output.

The term “transparent waveguide” as used herein means that a waveguidetransmits most incident solar light and/or light emitted by thephotoluminescent material. More particularly, the term “transparentwaveguide” means that said waveguide transmits at least 50%, preferablyat least 70% of said light measured for light incident perpendicular tothe waveguide.

The terms “ordinary refractive index” and “extraordinary refractiveindex” as used herein refer to the refractive indices of an alignedpolymer perpendicular and parallel to the optic axis of the alignedpolymer, respectively.

The term “refractive index of the waveguide” refers to the refractiveindex of the waveguide in the isotropic state. In specific cases,oriented waveguides may be used which exhibit birefringence due to, forinstance, flow during the production process.

The term “polymer layer” as used herein encompasses polymeric materialsin the form of sheets, strips, bands, fibres, ribbons, woven andstrands. The invention is not restricted to flat polymer layers andincludes polymer layers that have been bent, moulded or otherwiseshaped, provided the aligned polymer within the polymer layer isoriented at a pretilt angle relative to the surface of the object asdefined above.

The terms “aligned” and “oriented” as used herein in relation topolymers, photoluminescent materials or to groups contained in thesepolymers or materials, are synonyms and indicate that amongst thesepolymers, materials or groups a particular spatial orientation isprevailing.

By the terminology “aligned polymer having a pretilt angle of 10-90°” itis meant that the mesogenic groups of the aligned polymer are orientedat a pretilt angle of at least 10-90° relative to the surface of theluminescent object.

The term “liquid crystal” or “mesogen” is used to indicate materials orcompounds comprising one or more (semi-) rigid rod-shaped,banana-shaped, board-shaped or disk-shaped mesogenic groups, i.e. groupswith the ability to induce liquid crystal phase behaviour. Liquidcrystal compounds with rod-shaped or board-shaped groups are also knownin the art as ‘calamitic’ liquid crystals. Liquid crystal compounds witha disk-shaped group are also known in the art as ‘discotic’ liquidcrystals. The compounds or materials comprising mesogenic groups do notnecessarily have to exhibit a liquid crystal phase themselves. It isalso possible that they show liquid crystal phase behaviour only inmixtures with other compounds, or when the mesogenic compounds ormaterials, or the mixtures thereof, are polymerized.

For the sake of simplicity, the term “liquid crystal material” is usedhereinafter both to describe liquid crystal materials and mesogenicmaterials, and the term ‘mesogen’ is used for the mesogenic groups ofthe material. The compounds or materials comprising mesogenic groups donot necessarily have to exhibit a liquid crystal phase themselves. It isalso possible that they show liquid crystal phase behaviour only in adefinite (polymerised) layer, e.g. a coating layer on a waveguide (seebelow).

The term “liquid crystalline monomer” as used herein refers to amaterial which can undergo polymerization thereby contributingconstitutional units to the essential structure of a liquid crystallinepolymer.

The term “reactive liquid crystalline monomer” as used herein refers toa liquid crystalline monomer that contains a reactive group that can bepolymerized to form a liquid crystalline polymer or liquid crystallinepolymeric network.

The term “liquid crystalline polymer” as used herein refers to a polymermaterial in a mesomorphic state having long-range orientational orderand either partial positional order or complete positional disorder(IUPAC Recommendations 2001; Pure Appl. Chem. (2002) 74(3), 493-509).

The term “dichroic ratio” as used herein refers to the dichroic ratioderived from polarization selective absorption of the photoluminescentmaterial.

The dichroic ratio is derived from absorption measurements usinglinearly polarized light with the equation below:R=A _(//) /A _(⊥).In this equation R is the dichroic ratio, A_(//) the absorbance of thesample with the electric field of the incident light aligned parallel tothe alignment direction induced by the aligned polymer, and A_(⊥) theabsorbance of the sample with the electric field of the incident lightaligned perpendicular to the alignment direction induced by the alignedpolymer. The dichroic ratio of a photoluminescent material can bedetermined by means of different techniques well-known in the art, thesuitability of which techniques depends on the nature of thephotoluminescent material and of the aligned polymer matrix in which itis contained.

The phrase “homeotropically aligned photoluminescent polymer coating”refers to a polymeric coating comprising photoluminescent material,wherein the pretilt angle is 90°.

The term “pretilt angle” of the alignment refers to an angle made withthe horizontal, for instance the surface of an upper layer, and is knownto the person skilled in the art.

DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a cross-section of an optical laminate comprising aluminescent layer 1 and a waveguide 2. The laminate furthermorecomprises two wavelength selective and polarisation selective reflectingcholesteric layers 7 and 8, as well as a reflective mirror layer 9.Optical radiation, represented by arrows 4, enters the laminate throughthe left handed cholesteric layer 8 and the right handed cholestericlayer 7 and reaches the luminescent layer 1. Within the luminescentlayer 1 photoluminescent molecules 3, which have been aligned at apretilt, are excited by the incident radiation 4 and emit opticalradiation depicted by the arrows 5. A large fraction of the emittedradiation 5 enters the waveguide 2 where it is internally reflecteduntil it reaches the exit 10 or 11. The cholesteric layers 7 and 8ensure that not more than a tiny fraction of the emitted radiation 5will escape the laminate as together these wavelength-selective mirrors7 and 8 effectively reflect emitted radiation that re-enters theluminescent layer 1. The mirror layer 9 reflects optical radiation 4 andemitted radiation 5 back into the waveguide.

FIG. 2 depicts a cross-section of an optical laminate that is identicalto the one shown in FIG. 1, except that the mirror layer 9 is replacedby two cholesteric layers 9 a and 9 b. By employing a combination ofcholesteric layers 7, 8, 9 a and 9 b that is largely reflective to theemitted radiation 5 but transparent to optical radiation 12 that is notabsorbed by the photoluminescent dye molecules 3, it can be ensured thatoptical radiation 12 will travel through the complete laminate. Thisparticular laminate may advantageously be employed as a windowpane thatselectively concentrates a fraction of the incident solar light whilstallowing the other fraction to travel through the pane. As mentionedabove, instead of two layers 9 a and 9 b, more or less layers can beused.

FIG. 3 depicts a cross-section of an optical laminate comprising aluminescent layer 1 and a waveguide 2. The laminate furthermorecomprises a polymeric stack layer 7 comprising a plurality of sub-layers7 a to 7 k, as well as a reflective mirror layer 9. Optical radiation,represented by arrows 4, enters the laminate through the polymeric stacklayer 7 and reaches the luminescent layer 1. Within the luminescentlayer 1 tilt-aligned photoluminescent molecules 3 are excited by theincident radiation 4 and emit optical radiation depicted by the arrows5. A large fraction of the emitted radiation 5 enters the waveguide 2where it is internally reflected until it reaches the exit 10. Thepolymeric stack layer 7 ensures that not more than a tiny fraction ofthe emitted radiation 5 will escape the optical laminate as thiswavelength-selective mirror 7 effectively reflects emitted radiationthat re-enters the luminescent layer 1. The mirror layer 9 reflectsoptical radiation 4 and emitted radiation 5 back into the waveguide 2.Three sides of the optical laminate, including exit 11 are also providedwith a reflective mirror coating 9 that reflects both optical radiation4 and emitted radiation 5 back into the waveguide 2. Thus, effectivelythe only exit for emitted radiation from the optical laminate is theexit 10 which is optically coupled to a photovoltaic cell 13

FIG. 4 depicts a cross section of an optical fibre comprising aluminescent layer 1 and a waveguide core 2. The laminate furthermorecomprises two wavelength selective and polarisation selective reflectingcholesteric layers 7 and 8. Optical radiation, represented by arrows 4,enters the laminate through the left handed cholesteric layer 8 and theright handed cholesteric layer 7 and reaches the luminescent layer 1.Within the luminescent layer 1 pretilt aligned photoluminescentmolecules 3 are excited by the incident radiation 4 and emit opticalradiation depicted by the arrows 5. A large fraction of the emittedradiation 5 enters the waveguide 2 where it is internally reflecteduntil it reaches the exit 10. The cholesteric layers 7 and 8 ensure thatnot more than a tiny fraction of the emitted radiation 5 will escape theoptical fibre as together these wavelength-selective mirrors 7 and 8effectively reflect emitted radiation that re-enters the luminescentlayer 1.

FIG. 5 a depicts a cross section of an optical fibre comprising aluminescent core 1 and a waveguide 2. The laminate furthermore comprisestwo wavelength selective and polarisation selective reflectingcholesteric layers 7 and 8. Optical radiation, represented by arrows 4,enters the laminate through the left handed cholesteric layer 8 and theright handed cholesteric layer 7 and reaches the luminescent core 1.Within the luminescent layer 1 pretilt aligned photoluminescentmolecules 3 are excited by the incident radiation 4 and emit opticalradiation depicted by the arrows 5. A large fraction of the emittedradiation 5 enters the waveguide 2 where it is internally reflecteduntil it reaches the exit 10. The cholesteric layers 7 and 8 ensure thatnot more than a tiny fraction of the emitted radiation 5 will escape theoptical fibre as together these wavelength-selective mirrors 7 and 8effectively reflect emitted radiation that hits the outside wall of thewaveguide 2.

FIG. 5 b depicts a cross section of an optical fibre comprising aluminescent core 1. The laminate furthermore comprises a wavelengthselective and polarisation selective reflecting cholesteric layer, inthis embodiment two layers 7 and 8. The cholesteric layer(s) 7 and 8ensure that not more than a tiny fraction of the emitted radiation 5will escape the optical fibre as together these wavelength-selectivemirrors 7 and 8 effectively reflect emitted radiation that hits theoutside wall of the core 1. The photoluminescent material may be aligned(as depicted) or may be isotropically arranged (see also below).

FIG. 6 depicts a cross section of an optical laminate comprising aluminescent layer 1 and a waveguide 2. The luminescent layer 1 containsa plurality of isotropically arranged photoluminescent dye molecules 3.Incident optical radiation 4 enters the luminescent layer 1, where itexcites the photoluminescent dye molecules 3 into emitting opticalradiation 5 in all directions. As can be seen from FIG. 1 a significantfraction of the emitted radiation leaves the luminescent layer 1 throughthe top surface 6, thus reducing the efficacy with which the laminate iscapable of concentrating the incident radiation.

FIG. 7 depicts a cross section of an optical laminate that is identicalto the one depicted in FIG. 6, except that the plurality ofphotoluminescent dye molecules 3 has been aligned at a relatively smallangle of pre-tilt α. The photoluminescent dye molecules 3 emit opticalradiation 5 largely in a direction perpendicular to the pre-tiltalignment. As shown in the figure radiation is emitted at a relativelylarge angle relative to interface 14 between the luminescent layer 1 andthe waveguide 2, allowing a large fraction of said radiation to becoupled into the waveguide.

FIG. 8 depicts a cross section of an optical laminate that is identicalto the one depicted in FIG. 6, except that the plurality ofphotoluminescent dye molecules 3 has been tilt-aligned at a relativelylarge pre-tilt angle α. The photoluminescent dye molecules 3 emitoptical radiation 5 largely in a direction perpendicular to thetilt-alignment. Consequently, a significant fraction of the emittedradiation 5 will hit the interface between the luminescent layer 1 andthe waveguide 2 at an angle that is well above the reflection angle,meaning that most of this radiation will be reflected at the interface14 of the luminescent layer 1 and the waveguide 2.

FIG. 9 presents results of example 1.

FIG. 10 schematically depicts a measurement setup as used herein.

FIG. 11 presents results of example 2.

FIG. 12 schematically depicts a measurement setup as used herein.

FIGS. 13 a-13 c schematically depict a number of general embodimentsaccording to the invention, wherein luminescent layer 1 contains aplurality of isotropically arranged photoluminescent dye molecules 3.FIG. 13 a schematically depicts a general embodiment with luminescentlayer 1 and a wave-length selective mirror comprising a wavelengthselective mirror 7 (or 8), which preferably comprises a cholestericlayer, reflecting left-handed or right-handed circularly polarizedlight. FIG. 13 b schematically depicts an embodiment comprising twoselective mirror layers 7 and 8, preferably comprising cholestericlayers which preferably reflect left and right handed circularlypolarized light, respectively. FIG. 13 c schematically depicts anembodiment with a number of wavelength selective mirrors 7 (or 8), alllayers being reflective to circular or linear polarised light.Preferably, these layers comprise cholesteric layers which preferablyall reflect left or right handed circularly polarized light, i.e. anembodiment with a number of cholesteric layers each reflecting left orright handed circularly polarized light. As will be clear to the personskilled in the art, combinations of these embodiments are possible. Notdrawn, but in addition to the luminescent layer 1, there may be awaveguide 2, and one or more mirrors 9, 9 a, 9 b, or other layers, asdepicted in FIGS. 1-5 a, 14 a-e and 16 a-16 c. For instance, theluminescent layer 1 in FIGS. 1-ba, 14 a-e and 16 a-c, may instead of thedepicted alignment contain a plurality of isotropically arrangedphotoluminescent dye molecules 3. This means that layer 1 in theembodiments as for instance depicted in FIGS. 1-5 b, 14 a-e, 15 and 16a-c, may contain instead of aligned photoluminescent molecules 3,contain isotropically arranged photoluminescent molecules 3. In case noseparate waveguide 2 is present, the photoluminescent molecules arepreferably isotropically arranged; in case a waveguide 2 is presentisotropically or aligned photoluminescent molecules 3 may be present.

FIGS. 14 a-14 e schematically depicts embodiments of devices accordingto the invention comprising an LSC and a photovoltaic cell 13. Insteadof mirror 9, also one or more cholesteric layers may be used (see alsoFIG. 5).

FIG. 15 schematically depicts an embodiment of a device according to theinvention comprising a number (i.e. here 2 or more) LSC's and aphotovoltaic cell 13. Optical coupling may be achieved via waveguides(for instance optical fibres) 26. The radiation from waveguides 2 mayoptionally be collimated into waveguides 26 by collimators 25.

FIGS. 16 a (mirror 9 or one or more cholesteric layers not included) and16 b schematically show variants on FIGS. 1 and 14 b (but may also beapplied as variants to other embodiments). FIG. 16 c schematically showsanother variant on FIG. 1 (but may also be applied as variant to otherembodiments).

The schematic figures herein do not exclude the presence of otherelements like for instance alignment layers to align one or morecholesteric layer(s) or alignment layers to produce the alignment of thealigned polymeric layer(s), as will be clear to the person skilled inthe art.

Further, referring to FIGS. 1-5 a and 14 a-14 e, the invention is alsodirect to embodiments wherein the position of the waveguide 2 andluminescent layer 1 are exchanged. For instance referring to FIG. 1,this would provide a stack with the following sequence: cholestericlayer 8, cholesteric layer 7, waveguide 2, luminescent layer 1,reflective mirror 9 (see FIG. 16 a). Referring to for instance FIG. 14a, this would provide a stack with the following sequence: cholestericlayer 8, cholesteric layer 7, waveguide 2, alignment layer 20,luminescent layer 1, reflective mirror 9 (see FIG. 16 b). Likewise thisapplies to the other described and schematically depicted embodimentsherein.

DETAILED DESCRIPTION OF THE INVENTION

A luminescent solar cell is also described in DE 2737847, comprisingcells containing fluorescent solutions alternated with cells containinggas (air). However, the LSC of DE 2737847 does neither comprise awaveguide nor disclose an oriented photoluminescent material beingimmobilized within the aligned polymer, wherein the aligned polymer hasa pretilt angle of 10-90° relative to the surface of the luminescentobject. Further, D1 describes the use of interference filters, but doesnot describe the use of a cholesteric material as wavelength selectivemirror. Hence, the LSC, or more precisely, the luminescent object of DE2737847 does not provide the herein described advantages of the LSC andluminescent object, respectively. Luminescent layers andwavelength-selective layers may also be known from US2002/07035,EP0933655 and US2004/0105617, but none of these documents describe aluminescent layer and a wavelength selective mirror comprising acholesteric material.

A first aspect of the invention relates to a luminescent objectcomprising:

-   a. a luminescent layer or core containing a photoluminescent    material; and-   b. a wavelength-selective mirror;-   wherein the luminescent layer or luminescent core is optically    coupled to the wavelength-selective mirror, said    wavelength-selective mirror being at least 50% transparent to light    absorbed by the photoluminescent material and at least 50%    reflective to radiation that is emitted by the photoluminescent    material.

It is an essential aspect of the present object that the reflectivity ofthe wavelength-selective mirror for radiation emitted by thephotoluminescent material substantially exceeds the reflectivity of thesame mirror for optical radiation absorbed by said photoluminescentmaterial. Preferably the reflectivity for emitted radiation exceeds thereflectivity for absorbed radiation by at least 50%, more preferably byat least 80% and most preferably by at least 100%.

In order to enjoy full advantage from the wavelength-selective mirrorsaid mirror should cover at least 80% of one side of the luminescentlayer or at least 80% of the exterior surface of the luminescent core.Furthermore, it is preferred to employ a relatively thinwavelength-selective mirror. Typically, the thickness of thewavelength-selective mirror does not exceed 100 μm, preferably it doesnot exceed 20 μm. Usually, the thickness of the aforementioned mirrorwill exceed 5 μm. It is noted that the wavelength-selective mirror ofthe present invention may suitably comprise two or more layers thattogether function as a wavelength selective mirror, e.g. a polymericstack or a combination of cholesteric layers.

The efficiency of the aforementioned arrangement is dependent on boththe transparency of the wavelength-selective mirror for light absorbedby the photoluminescent material and the reflectivity of the same mirrorfor the emitted radiation. Preferably, both parameters are maximised, beit that in practice it is difficult to optimise both parametersindependently. It is feasible to provide a wavelength-selective mirrorthat is at least 60%, preferably 70% transparent, more preferably atleast 80% and most preferably at least 90% transparent to light that isabsorbed by the photoluminescent materials. Furthermore, it is possibleto provide a wavelength-selective mirror that is at least 60%,preferably 70%, more preferably at least 90% reflective to radiationthat is emitted by the photoluminescent material.

The efficiency with which the present object concentrates radiationemitted by the photoluminescent material critically depends on theefficiency with which the wavelength-selective mirror reflects saidradiation. Typically, the wavelength-selective mirror exhibits a maximumreflectivity of at least 50%, preferably of at least 60%, morepreferably of at least 70% for optical radiation with a wavelengthwithin the range of 500-2000 nm, preferably within the range of 600-2000nm, and most preferably within the range of 630-1500 nm.

Likewise, and in particular if the wavelength-selective mirror ispositioned as a separate layer anywhere between the luminescentlayer/core and the surface that is meant to receive incident opticalradiation, it is important that high-energetic radiation that is capableof exciting the photoluminescent material is transmitted by said mirrorwith high efficiency. Accordingly, the wavelength-selective mirrorpreferably exhibits a maximum transmittance of at least 60%, preferablyof at least 70% for optical radiation with a wavelength within the rangeof 350-600 nm, preferably within the range of 250-700 nm, and even morepreferably within the range of 100-800 nm.

Since the wavelength of radiation emitted by the photoluminescentmaterial will inevitably exceed the wavelength of the radiation absorbedby the same material, it is preferred that the maximum of reflectivityoccurs at a wavelength that exceeds the transmittance maximum,preferably by at least 30 nm, more preferably by at least 50 nm, evenmore preferably by at least 100 nm.

The wavelength-selective mirror employed in the present object mayadvantageously comprise a polymeric wavelength selective mirror and/or apolarization selective mirror.

In an advantageous embodiment, the wavelength-selective mirror comprisesa polarization selective mirror which is at least 50%, preferably atleast 60%, more preferably at least 70% transparent to light absorbed bythe photoluminescent material and that is at least 50%, preferably atleast 70% reflective to circular or linear polarized radiation with theappropriate polarisation, notably to circular or linear polarizedradiation that is emitted by the photoluminescent material. Such anadvantageous arrangement may be realised by employing awavelength-selective mirror comprising polymeric stack layers and/orcholesteric layers.

The present luminescent object may advantageously contain awavelength-selective mirror that comprises a cholesteric layer of chiralnematic polymer. In an even more preferred embodiment the polymericwavelength-selective mirror comprises a first cholesteric layerreflecting right-handed circularly polarized light and a secondcholesteric layer reflecting left-handed circularly polarized light. Inthe latter embodiment the luminescent layer is suitably sandwichedbetween the cholesteric layers and a waveguide or, alternatively, awaveguide is sandwiched between the cholesteric layers and theluminescent layer. Preferably, the luminescent layer is sandwichedbetween the adjacent cholesteric layers and the waveguide. Cholestericlayers are capable of effectively reflecting a narrow band of circularlypolarised radiation. Depending on the helical orientation of thecholesteric layer the layer will reflect either right-or left-circularlypolarised radiation. By employing two cholesteric layers with oppositehelical orientations, both right- and left-circularly polarised lightwill be reflected effectively.

The present luminescent object may also advantageously contain awavelength-selective mirror that comprises one or more cholestericlayer(s) of chiral nematic polymer. Preferably, the polymericwavelength-selective mirror comprises one or more layers selected fromthe group consisting of a cholesteric layer reflecting right-handedcircularly polarized light and a cholesteric layer reflectingleft-handed circularly polarized light. In an embodiment, theluminescent aligned polymer layer may be sandwiched between thecholesteric layer(s) and the waveguide or the waveguide may besandwiched between the cholesteric layer(s) and the luminescent alignedpolymer layer. Preferably, the luminescent aligned polymer layer issandwiched between the adjacent cholesteric layers and the waveguide.

In specific variants, the polymeric wavelength-selective mirrorcomprises one or more cholesteric layer(s) reflecting right-handedcircularly polarized light or one or more cholesteric layer(s)reflecting left-handed circularly polarized light or comprises both oneor more cholesteric layer(s) reflecting right-handed circularlypolarized light and one or more cholesteric layer(s) reflectingleft-handed circularly polarized light. A “simple” right- andleft-handed two layer system may for instance only reflect a 75 nmbandwidth of light. It is possible to broaden the band, but in the artthis appears not to be simple. According to the invention, it mayadvantageously be easier to broaden the band of wavelengths reflected bylayering successive right-handed cholesterics on top of each other,followed by left-handed on top of each other, or vice versa, or anycombination of right- and left-handed layers. It is also conceivable touse only one handedness of cholesterics for the whole sample, i.e. forinstance 2-5 left handed layers or 2-5 right handed layers. Theinvention is not restricted to a 2-layer system.

A chiral substance mixed with a nematic material induces a helical twisttransforming the material into a chiral nematic material, which issynonymous to a cholesteric material. The cholesteric pitch of thechiral nematic material can be varied over a rather large range withcomparative ease. The pitch induced by the chiral substance is, in afirst approximation, inversely proportional to the concentration of thechiral material used. The constant of proportionality of the thisrelation is called the helical twisting power (HTP) of the chiralsubstance and defined by the equation:HTP=1/(c·P)wherein c is the concentration of the chiral substance and P is theinduced helical pitch.

Optically active compounds that are capable of inducing a helicalstructure are generally referred to as “chiral dopant”. Many chiraldopants have been synthesised, and typical examples thereof includecompounds represented by the following structure:

2,2-dimethyl-4,5-diphenyl-1,3-Dioxolane

ZLI 811, Benzoic acid, 4-hexyl-, 4-[[(1-methylheptyl)oxy]carbonyl]phenylester (9CI)

The cholesteric layer or combination of cholesteric layersadvantageously reflects optical radiation emitted by the luminescentlayer or core and is largely transparent to optical radiation with awavelength in the range of 100-600 nm, preferably of 250-700 nm and mostpreferably of 350-800 nm.

In another embodiment, the present luminescent object additionallycomprises a wavelength-selective mirror in the form of polymeric stacklayer that is strongly reflective to radiation that is emitted by thephotoluminescent material. More particularly, the polymeric polarizationselective mirror comprises a first polymeric stack layer reflecting oneplane of polarized light and a second polymeric stack layer reflectingthe opposite plane of polarized light, wherein a luminescent layer issandwiched between the polymeric stack layers and a waveguide or whereina waveguide is sandwiched between the polymeric stack layers and theluminescent layer.

In a specific embodiment, there is provided a luminescent object,wherein the wavelength-selective mirror comprises a first polymericstack layer reflecting one polarization of light and a second polymericstack layer reflecting the opposite polarization of light, bothpolymeric stack layers being located at the same side of the luminescentlayer or being located outside the luminescent core.

Polymeric stack layers are capable of selectively reflecting opticalradiation within a certain wavelength range. Polymeric stack layers arealso referred to as multilayer reflectors and are used to partitionportions of the electromagnetic spectra between reflection andtransmission. Polymeric stack layers typically employ a number of layersof at least two different materials within an optical stack. Thedifferent materials have refractive indices along at least one in-planeaxis of the stack that are sufficiently different to substantiallyreflect light at the interface of the layers. Polymeric stack layers canbe constructed to reflect optical radiation incident at normal and/oroblique angles of incidence.

Preferably, the polymeric stack layers employed in the presentluminescent object have been designed to reflect optical radiation above600 nm, more preferably above 700 nm and most preferably above 800 nm.In a preferred embodiment, the luminescent layer is sandwiched betweenthe polymeric stack layer and a waveguide.

Polymeric stack layers that are employed as wavelength-selective mirrorsin accordance with the present invention may suitably be prepared usingthe methodology described in U.S. Pat. No. 6,157,490 and Weber, M. F. etal. Science 287, 2451, which are incorporated herein by reference.

The photoluminescent material employed in accordance with the inventiontypically emits optical radiation with a wavelength in the range of 100nm to 2500 nm. Preferably, the photoluminescent material emits radiationin the range of 250-1500 nm, more preferably in the range of 400-1000nm. For many applications an optimum photoluminescent material has awide absorption range covering most of the solar spectrum as well as anarrow emission range having a somewhat longer wavelength. Thus, thephotoluminescent material absorbs incoming solar radiation and emits itat another wavelength. The photoluminescent material employed in thepresent object typically has an absorption curve with an absorptionmaximum below 800 nm, preferably below 700 nm, and most preferably below600 nm. According to a particularly preferred embodiment, the objectabsorbs light between 500 and 600 nm and emits light at a longerwavelength.

The inventors have discovered that the efficiency of the presentluminescent object as an element of an LSC-system can be increaseddramatically by employing therein a luminescent layer or luminescentcore comprising an aligned polymer that contains an orientedphotoluminescent material. The alignment of the polymer is used toinduce the orientation of the photoluminescent material.

Radiation emitted by non-aligned, isotropic photoluminescent materialtravels in all directions, with slight preference for an emissiondirection perpendicular to the LSC-system for illumination perpendicularto the plane of the LSC-system. In other words, a large fraction of thelight is emitted outside the waveguiding mode and is not transportedwithin the waveguide. The proper alignment of the photoluminescentmaterial within the polymer ensures that a large fraction of the lightemitted by the oriented photoluminescent material is radiated into thewaveguiding mode of the LSC-system.

Thus, in case of alignment at a significant pretilt angle, radiationwill be emitted at an angle that allows it to be coupled veryeffectively into a waveguide. Alternatively, in the case of alignment ata large pretilt angle a relatively large fraction of the emittedradiation will meet the luminescent object's interface with air with anangle greater than the critical angle of total reflection and remaininside the object. Consequently, the use of aligned photoluminescentmaterial makes it possible to contain a much higher fraction of theemitted light within a LSC-system than was feasible up until now. As aresult, the present invention makes it possible to increase theoperating efficiency of LSC systems by more than 25%.

Accordingly, a preferred embodiment of the invention relates to aluminescent object wherein the luminescent layer or luminescent corecomprises an aligned polymer that contains an oriented photoluminescentmaterial, said aligned polymer having a pretilt angle of 10-90°. In casethe emitted light is radiated by the photoluminescent material at arelatively small angle relative to the surface of the object (requiringthe use of a relatively high pretilt angle), the emitted light can betransported efficiently within the plane parallel to said surface to,for instance, an exit or a photovoltaic device. Thus, the present filmcan be applied as such, without a separate waveguide, in e.g. LSCs. Inthis particular case it is very advantageous to use photoluminescentmaterials with a large Stokes Shift and/or with little overlap betweenthe absorption and emission spectra to avoid large light losses byre-absorption phenomena.

In an embodiment, there is provided a luminescent object comprising aluminescent layer and a waveguide, wherein the object is an opticallaminate or an optical fibre, the luminescent object being coupledoptically to the waveguide, the luminescent object comprising an alignedpolymer that contains an oriented photoluminescent material, saidoriented photoluminescent material being immobilized within the alignedpolymer, and said aligned polymer having a pretilt angle of 10-90°relative to the surface of the object.

The luminescent object according to the invention can alsoadvantageously be coupled optically to a (transparent) waveguide(without fluorescent dyes) as the efficiency of transfer (orin-coupling) of emitted light into the waveguide is greatly enhanced bythe present invention, especially if the photoluminescent material isoriented at a pretilt angle within the range of 30-70°. The opticalcoupling of the present luminescent object can suitably be achieved byproducing a multi-layer structure (e.g. an optical laminate or amulti-layer optical fibre) in which the luminescent object is bonded asa separate layer onto the waveguide.

Typically, the photoluminescent material is aligned within theluminescent layer/core in essentially the same direction as themesogenic groups of the aligned polymer. Dichroic photoluminescentmaterials are particularly suitable for use in accordance with thepresent invention as they can be oriented relatively easily within amatrix of aligned polymer, e.g. liquid crystalline polymer.

The mesogen can be a reactive mesogen or a non-reactive mesogen.Examples of suitable non-reactive mesogens are those available fromMerck™, for example as described in their product folder Licristal®Liquid Crystal Mixtures for Electro-Optic Displays (May 2002) whosecontents regarding non-reactive mesogens are incorporated herein byreference.

Examples of suitable reactive mesogens are those comprising acrylate,methacrylate, epoxy, vinyl-ether, styrene, thiol-ene and oxethanegroups. Suitable examples are for example described in WO04/025337 whosecontents regarding reactive mesogens, referred in WO04/025337 aspolymerizable mesogenic compounds and polymerizable liquid crystalmaterials, are incorporated herein by reference. Also mixtures ofreactive mesogens can be used (Merck™ Reactive Mesogens, Brighterclearer communication).

Also mixtures of reactive and non-reactive mesogens can be used. In caseof a mixture, all mesogens used are preferably in an aligned state inthe final layer.

In the case of liquid crystalline polymers it is advantageous toincorporate and/or dissolve the fluorescent material in a liquidcrystalline reactive monomer. These monomers easily align in thepresence of a field (flow, magnetic, electrical, poling, mechanicaldrawing) or in the presence of alignment layers (buffed or non-buffedpolyimides, linear photopolymerizable materials, etc.). The pretiltwhich is generated can be easily controlled by those skilled in thestate of the art (see for example Sinha et al (2001) Appl. Phys. Lett.79, 2543). After the appropriate alignment of the reactive monomer(mixture) a thermal, or radiation induced polymerisation of the liquidcrystalline monomer is performed. In specific cases, it is advantageousto add appropriate polymerisation initiators. For instance, in the caseof a polymerisation with ultra-violet light a UV-initiator (see forexample Irgacure 184, Ciba Specialty Chemicals) is used and in the caseof a thermal polymerisation a proper thermal initiator (see for example2,2′-azobisisobutyronitrile (AIBN), Aldrich Chemicals) is used.

It is possible to discern several kinds of pretilt orientations.According to a typical example of a pretilt orientation, the director ofliquid crystal molecules in a liquid crystal layer is almost identicalat any position in the film thickness direction. It is also feasible toprovide an orientation wherein in the vicinity of one of the layer'ssurfaces the director is generally parallel to said surface, and as theopposite surface of the layer is approached, the director graduallychanges, exhibiting a homeotropic orientation or an oriented state closethereto (splay configuration). Both a tilt orientation wherein the anglebetween the director and a projection of the director to a plane of thelayer is constant at any point in the layer thickness direction, and anorientation wherein the said angle changes continuously in the layerthickness direction, are included in the scope of the pretiltorientation as referred to herein.

In a preferred embodiment, the present luminescent object comprises atleast one layer of the aligned polymer containing orientedphotoluminescent material, wherein the top surface of said layercoincides with or extends parallel to the top surface of the luminescentobject. Such a luminescent object can be employed to concentrateincident optical radiation in a highly efficient manner.

The present luminescent object may comprise one or more layerscomprising aligned polymer and oriented photoluminescent material. Theuse of several such layers offers the advantage that each of the layerscan be optimised to absorb a particular bandwidth of optical radiationso that the overall film is capable of absorbing and concentrating awide spectrum of optical radiation. The use of several layers alsoallows different polymers to be used in each layer as dictated by thepreferences of individual photoluminescent dyes or the necessity ofachieving specific tilt-alignments.

The photoluminescent material employed in the luminescent object maysuitably have been mixed into the aligned polymer by doping the alignedpolymer with the oriented photoluminescent material. Alternatively, theoriented photoluminescent material can be covalently bonded to thealigned polymer. In accordance with yet another suitable embodiment ofthe invention, the oriented photoluminescent material is a mesogenicgroup of the aligned polymer.

The oriented photoluminescent material in the present object may consistof a single photoluminescent component or it may comprise a mixture ofphotoluminescent components. It can be advantageous to employ acombination of photoluminescent components that each absorb opticalradiation of different wavelengths. Thus, by selecting a suitablecombination of photoluminescent components it can be ensured that thephotoluminescent material contained in the present object absorbs a wideband of optical radiation, e.g. a major part of the solar radiationspectrum. In case the present object contains a plurality of layers, itmay be advantageous to apply different photoluminescent materials indifferent layers. Thus, the performance of the present object in termsof light concentrating efficiency may be maximised. Naturally, if acombination of photoluminescent components is used, care must be takento ensure that there is little or no overlap between the wavelengths atwhich this combination of photoluminescent components emits and absorbsradiation or, in case there is strong overlap, the combination shouldact as a cascade, meaning that radiation emitted by one photoluminescentcomponent and absorbed by another component will cause the lattercomponent to luminesce.

The oriented photoluminescent material employed in the luminescentobject of the present invention preferably has a dichroic ratio of atleast 2.0, more preferably of at least 3.0, most preferably of at least5.0. In a planar orientation, dichroic photoluminescent material willabsorb one linear polarisation direction of optical radiation to asubstantially greater extent than the other ones.

Dichroic photoluminescent materials are particularly suitable for use inthe present luminescent object. According to a preferred embodiment, theoriented photoluminescent material comprises organic and/or polymericphotoluminescent dyes. As used herein, the term “photoluminescent dye”means a dye which is a molecule that colours by itself, and thus absorbslight in the visible spectrum and possibly in the ultraviolet spectrum(wavelengths ranging from 100 to 800 nanometers), but which, in contrastwith a standard dye, converts the absorbed energy into fluorescent lightof a longer wavelength emitted primarily in the visible region of thespectrum. The photoluminescent dyes should possess a high quantumefficiency, good stability, and be highly purified. The dyes are usuallypresent in a concentration of from 10⁻¹ to 10⁻⁵ Molar. Typical examplesof organic photoluminescent dyes that can suitably be employed inaccordance with the present invention include, but are not restrictedto, substituted pyrans (such as DCM), coumarins (such as Coumarin 30),rhodamines (such as Rhodamine B), BASF® Lumogen™ series, perylenederivatives, Exciton® LDS series, Nile Blue, Nile Red, DODCI, oxazines,pyridines, the ‘styryl’ series (Lambdachrome®), dioxazines,naphthalimides, thiazines, and stilbenes.

It is an essential aspect of the present invention that the orientedphotoluminescent material is immobilised within an aligned polymermatrix. An object based on liquid crystalline polymer and containing aphotoluminescent material can be aligned in several ways. In the case ofliquid crystalline polymers it is often preferred to align a reactiveliquid crystalline monomer and subsequently polymerize the monomer asdiscussed previously. This procedure is usually adopted because liquidcrystalline monomers easily orient (in contrast to most liquidcrystalline polymers).

The surface of the object is usually provided with an orientation layerwhich induces the proper alignment of the liquid crystalmonomer/polymer. Some possible orientation layers are:

-   a. Polyimide alignment layers (buffed, rubbed or non-buffed,    non-rubbed) are conventionally used to generate aligned liquid    crystalline polymers with a planar or homeotropic alignment and/or    with a specific pretilt. Typical examples are Optimer A1 1051, (ex    JSR Micro) for planar alignment and 1211 polyimide varnish (ex.    Nissan Chemical) for homeotropic alignment:-   b. So called linear photopolymerizable materials (LPP) can be used    as an alignment layer with a well-defined pretilt (see for example    Staralign™, Vantico AG, Basel, CH)    Other techniques for the alignment of liquid crystals include:-   a. Recording with a sharp or blunt stylus, oblique evaporation or    sputtering of SiO₂, glancing angle deposition of inorganics,    Langmuir-Blodgett deposited copper phthalocyanines doped    polymethacrylate layers, and diamond-like carbon thin film layers    (see for example references: Varghese et al (2004) Appl. Phys. Lett.    85, 230; Motohiro, T. and Taga, Y. (1990) Thin Solid Films 185, 137;    Castellano, J. A. (1984) 4, 763; Robbie, K. et al (1999) Nature 399,    764, Lu, R. et al (1997) Phys. Lett. A 231, 449, Hwang, J-Y et    al (2002) Jpn. J. Appl. Phys. 41, L654).-   b. Alignment inducing surfactants, e.g. silanes, higher alcohols    (e.g. n-dodecanol), and the like can be used to further tune the    alignment of the liquid crystals.-   c. By adding an alignment inducing dopant to the liquid crystalline    polymer.-   d. By applying mechanical drawing, flow, magnetic, electric poling    field to the object.-   e. By aligning polymers with smectic-A orientation through passing    the object over heated rollers. The resulting shear deformation    causes the mesogenic groups to become oriented.-   f. By aligning liquid crystals by using reactive mesogens that    attain smectic-C orientation by a (proper) heat treatment followed    by initiating the polymerization reaction of the mesogens to trap    the system in the smectic-C orientation.    Techniques that are particularly suitable for preparing a    luminescent object comprising an aligned polymer having a pretilt of    30-80° are described in Hwang, Z. and Rosenblatt, C. Appl. Phys.    Lett. 86, 011908, Lu, M. Jpn. J. Appl. Phys. 43, 8156, Lee, F. K. et    al Appl. Phys. Lett. 85, 5556, The Staralign™ linearly    photopolymerizable polymer system (Vantico AG), Varghese, S. et al.    Appl. Phys. Lett. 85, 230, and Sinha, G. P. et al. Appl. Phys. Lett.    79, 2543. These publications are incorporated herein by reference.

The benefits of the present invention are particularly pronounced inluminescent polymeric objects, notably flat objects, in which thealigned polymer is aligned at a pretilt angle of less than 85°,preferably of less than 80°. Preferably, the pretilt angle is 10-89°,more preferably 10-85°, even more preferably 15-85°; more preferably30-80°; preferably in the range of 30-70°, more preferably in the range35-65° and most preferably in the range of 40-60°.

In particular when used in flat luminescent objects in combination witha waveguide, it is advantageous to employ aligned polymer at apretilt-angle within the range of 10-89°, more preferably 10-85°, evenmore preferably 15-85°, more preferably in the range of 30-80°;preferably in the range of 30-70°, more preferably in the range 35-65°and most preferably in the range of 40-60°. The application of such apretilt-angle enables highly efficient in-coupling of the emittedradiation into the waveguide. As explained herein before the benefits ofthe present invention result from the alignment of the photoluminescentmaterial. The alignment of the photoluminescent material is achieved byimmobilising the photoluminescent material within an aligned polymermatrix. The prevailing orientation of the photoluminescent materialwithin such an aligned polymer matrix coincides with the alignment ofsaid polymer matrix. It will be understood therefore that the preferredorientation angles for the photoluminescent material are the same asthose mentioned above in relation to the aligned polymer.

In case the present luminescent object is a non-flat object, inparticular if said object is an optical fibre, in an embodiment apretilt angle of more than 50°, especially of more than 70° can beadvantageous. Most preferably, in case of the present object is anoptical fibre, the pretilt angle exceeds 80°. However, in another yetmore preferred embodiment, the pretilt angle is in the range of 30-70°,more preferably in the range 35-65° and most preferably in the range of40-60°.

The level of alignment of the photoluminescent material within a planarobject can suitably be defined in terms of the order parameter. Theorder parameter is defined as:S=(A _(//) −A _(⊥))/(A _(//)+2 A _(⊥))

Wherein A_(//) denotes amount absorption by the sample of light withelectric vector parallel to the alignment direction, A_(⊥) theabsorption by the sample of light with electric vector perpendicular tothe alignment direction, and S the average orientation of the absorptionmoment in the fixed laboratory frame. Reference: Van Gurp, M. AndLevine, Y. K., J. Chem. Phys. 90, 4095 (1989).

The photoluminescent material contained in the present object,preferably, exhibits an order parameter of at least 0.5, more preferablyof at least 0.6, most preferably of at least 0.7.

The efficacy with which the present luminescent object may be employedin e.g. LSC-systems is critically dependent on the level of(re-)absorption of emitted light within the same object. According to aparticularly preferred embodiment, the absorption maxima and emissionmaxima of the photoluminescent material contained within a discretealigned polymer matrix, e.g. a layer, differ by at least 30 nm,preferably by at least 50 nm, more preferably by at least 100 nm.

The luminescent object according to the invention advantageously takesthe shape of a film, a layer, a fibre, a ribbon, or woven. The thicknessof such film, layer, fibre, ribbon or woven may vary depending on theintended application. Typically, said thickness will be in the range of0.1-500 μm, preferably in the range of 5-50 μm.

The luminescent object advantageously comprises two or more stackedlayers of aligned polymer containing oriented photoluminescent material.In accordance with one particular embodiment each aligned polymer layercontains a different photoluminescent material. Advantageously, thesephotoluminescent materials exhibit different absorption maxima. Inaccordance with another embodiment, the pretilt angle of the alignedpolymer in the respective aligned polymer layers changes from layer tolayer. This arrangement enables further optimisation of the efficiencywith which incident radiations can be converted into photoluminescentradiation and subsequently be transported, especially through a separatewaveguide. In yet another embodiment, each aligned polymer layercontains a different aligned polymer. The use of stacked layers ofdifferent aligned polymers is particularly advantageous in combinationwith the application of different photoluminescent materials in each ofthe aligned polymer layers and/or in combination with the use of alignedpolymer layers that exhibit different pretilt angles.

The luminescent object of the present invention may advantageously beapplied onto e.g. windows, especially if these windows have been coupledto a photovoltaic device or if they are optically coupled to a source ofinternal lighting. By fixating the present luminescent object onto sucha window (or a building element with a similar function), the windoweffectively becomes a waveguide that concentrates the electromagneticradiation that is emitted by the luminescent layer. As will be explainedbelow, in these applications it is highly desirable that the luminescentobject is transparent to at least a fraction (e.g. across a bandwidth ofat least 100 nm) of the visible light spectrum, especially the part ofthe visible light spectrum that is needed for photosynthesis.

A particularly advantageous embodiment of the invention relates to aluminescent object in the form of an optical laminate or an opticalfibre, comprising a waveguide that is optically coupled to theluminescent layer or luminescent core, wherein the refractive index ofthe waveguide, n_(w), is such that n_(w)≧n_(o)−0.005, wherein n_(o) isthe ordinary refractive index of the luminescent layer of core.According to an even more preferred embodiment n_(w)≧n_(o), preferablyn_(w)>n_(o). This advantageously promotes containment of the light inthe waveguide and decrease escape from light for instance back to theluminescent layer.

The optical laminates according to the present invention may suitably beemployed to concentrate incident optical radiation. Radiation incidenton the laminate is absorbed and re-emitted by the luminescent layer.This re-emitted radiation is coupled into the waveguide and guided byinternal reflection along the waveguide to an outlet surface. There-emitted radiation may exit the outlet surface, allowing the outletsurface to function as a light source. Alternatively, at the outletsurface the re-emitted radiation may be coupled into a device, such as aphotovoltaic device, that will convert the optical radiation intoanother form of energy.

The optical fibres according to the invention may be laterally excitedby optical radiation, following which emitted radiation will betransported to the ends of the fibre. The fibres of the presentinvention may advantageously be employed for concentrating opticalradiation and/or for transmitting optical information.

Because the optical laminates and fibres according to the invention willre-emit radiation in response to incident radiation, these laminates andfibres may suitably be used to convey optical information, especiallyoptical binary information. Since, however, the level of emittedradiation correlates to the intensity of incident radiation, the presentoptical laminates and fibres may also be used to convey analogueinformation.

According to yet another preferred embodiment the refractive index ofthe waveguide is between the ordinary and the extraordinary refractiveindices of the luminescent layer/core. The refractive index of thewaveguide typically is at least 1.4. Preferably the refractive index ofthe waveguide is within the range of 1.4-1.8, more preferably within therange of 1.4-1.7. The application of a waveguide with a high refractiveindex, especially relative to the refractive index of the luminescentfilm, ensures that the optical path of the emitted radiation within theluminescent film is reduced as the emitted radiation is efficientlycoupled into the waveguide. In order to ensure that the emitted light isefficiently coupled into the waveguide it is furthermore advantageousfor the surface of the luminescent layer (or core) and the surface ofthe waveguide to be adjacently joined together. The aligned polymerlayer and waveguide may suitably be joined by means of an adhesive,provided the adhesive is largely transparent to the emitted radiation.

The luminescent layer within the optical laminate or fibre typically hasa thickness of 0.1-500 μm, preferably of 5-50 μm. The waveguidetypically has a thickness of 0.05-50 mm, preferably of 0.1-10 mm. Incase the waveguide constitutes the core of an optical fibre of thepresent invention, the aforementioned ranges are applicable to thediameter of the waveguide.

The optical laminates and fibres of the present invention may suitablybe produced from flexible, elastic materials. The flexible laminates andfibres so obtained may, for example, be produced as rolls or can beapplied onto curved surfaces. The present invention also encompassesfibres and laminates that are relatively rigid, e.g. because they makeuse of a glass layer or core.

The present optical laminates and fibres can be manufactured in manydifferent ways well-known in the art. The laminates may be produced byfirst providing one layer (film), e.g. the waveguide, followed by insitu creation of the other layers, e.g. by in situ polymerisation orhardening. Alternatively, the individual layers may be pre-manufacturedindividually and subsequently be combined into a single laminate,optionally using adhesives and/or other bonding techniques. Naturally,it is also possible to employ combinations of these techniques. Similarapproaches can be used in the manufacture of the optical fibres of thepresent invention, except that in the case of fibres the layeringprocess will start with providing the core of the optical fibre.

Suitable materials for the transparent waveguide are largely transparentfor the emitted radiation that is to be conveyed through the waveguide.Suitable materials include transparent polymers, glass, transparentceramics and combinations thereof. Preferably the waveguide is made of atransparent polymer which may be thermosetting or thermoplastic. Thesepolymers may be (semi-)crystalline or amorphous. Suitable polymersinclude polymethyl methacrylates, polystyrene, polycarbonate, cyclicolefin copolymers, polyethylene terephtalate, polyether sulphone,cross-linked acrylates, epoxies, urethane, silicone rubbers as well ascombinations and copolymers of these polymers.

The optical laminate of the present invention may suitably take theshape of a flat planar plate. However, since the functionality of theoptical laminate is not essentially dependent on the form of thelaminate, also non-planar shapes are encompassed by the presentinvention.

In order to prevent radiation from escaping the present luminescentobject it can be advantageous to employ a mirror that is reflective to abroad range of optical radiation, said mirror being located at thebottom surface of the object, meaning that incident radiation thatpasses through the luminescent layer as well as optical radiationemitted by the same layer in the direction of the mirror will bereflected by said mirror. More particularly, this embodiment provides aluminescent object as defined herein before, wherein the bottom surfaceis covered with a mirror that is at least 80% reflective for wavelengthsof 450-1200 nm, preferably at least 90% reflective for wavelengths of450-1200 nm.

Radiation losses from the present luminescent object may be furtherminimised by applying mirrors to the sides of the object that are notsupposed to transmit radiation to e.g. a photovoltaic device.Accordingly, in a preferred embodiment at least one of the sides of theobject is covered with a mirror that is at least 80% reflective forwavelengths of 450-1200 nm, preferably at least 90% reflective forwavelengths 450-1200 nm. More preferably at least two sides and mostpreferably at least three sides of the object are covered with such amirror.

For certain applications it may be advantageous if the presentluminescent object is largely transparent for optical radiation within aparticular wavelength range. According to a particularly preferredembodiment, the luminescent object is predominantly transparent foroptical radiation in the range of 400-500 nm and/or 600-700 nm. Thus,the luminescent object may suitably be applied onto or used instead ofe.g. windows or greenhouse panes as visible light that is used inphotosynthesis will pass through the luminescent object whilst opticalradiation of other wavelengths may be concentrated and transportedeffectively by the same object. According to a very preferredembodiment, the luminescent object is predominantly transparent foroptical radiation in both the range of 400-500 nm and 600-700 nm. Thus,the present invention also encompasses a greenhouse comprising one ormore panes covered with an luminescent object as defined herein beforethat is transparent for optical radiation in the range of 400-500 and/or600-700 nm in combination with one or more photovoltaic cells capable ofconverting optical radiation to electrical energy, said one or morephotovoltaic cells being optically coupled to the luminescent object.

A further embodiment of the invention relates to a photovoltaic devicecomprising an electromagnetic radiation collection medium containingluminescent object as defined herein before and a photovoltaic cellcapable of converting optical radiation to electrical energy which isoptically coupled to a waveguide comprised by the luminescent object,said photovoltaic cell preferably being arranged at the edge of thewaveguide, or near the edge of the waveguide on the top or bottomsurface of the waveguide layer.

Another embodiment of the invention concerns a fluorescent lightactivated display comprising a luminescent object as defined above.

Yet another embodiment of the present invention relates to a roomlighting system comprising a luminescent object as defined above,wherein the system is arranged in such a way that incident solar lightis converted to room lighting by said luminescent object.

Another aspect of the invention relates to the use of a luminescentobject as defined herein before for concentrating incident opticalradiation. This use of the luminescent object encompasses e.g. the useof self-supporting luminescent films that comprise a luminescent layerand a wavelength-selective mirror as defined herein before. Furthermore,said use comprises the use of optical laminates or optical fibres thatcomprise a luminescent layer or core, a wavelength-selective mirror anda waveguide.

In an embodiment, there is provided a LSC system without an opaque (i.e.for instance without a rear mirror, as described herein) rear surfacefor implementation in a greenhouse. Preferably, in a further variant,the wavelength regions of the spectra not used (for growth) by theplants are collected and converted into electricity via thephotovoltaic. In yet another variant, the light of longer wavelength(i.e. the converted light), is collected and via a light pipe used tore-illuminate the plants at a wavelength the plants use for growth. Forinstance, the LSC system may be applied to collect and convert greenlight, which is in general not used by plants, and convert the lightinto red light (or blue light, in case upconverter materials are used).The generated red light may be used by the plants in the greenhouse.

In yet a further embodiment, LSC as fibres according to the inventioncould be woven into or onto clothing or other materials (clothes,sleeping bags, tents, et), and bundles of the fibres illuminating aphotovoltaic or solar cell for electrical generation (see for instancealso FIG. 15). The invention is also directed to such product asclothes, sleeping bags, tents, etc. (for instance with fibres of about0.005 mm-10 cm, preferably about 0.5 mm-1.0 cm). A luminescent objectsin the form of a laminate or sheet with an optical wave guide accordingto the invention can also be used to construct objects such as tentsiding, and can be used for generation of electricity by a solar cell.The invention is also directed to such objects.

Further, for instance small scale consumer products, such as laptopcovers, pens, watches, calculator covers, jewelry, hats, caps, etc.could contain sheets or fibres of the material designed for electricalgeneration by a solar cell.

It would also be possible to use the light output directly for visualeffects rather than electrical generation (i.e. the application orobject as above mentioned without a solar cell).

Further application of the invention may be, for instance, road signs ormarkers, to generate electricity, or to induce or enhance the visualsafety effects of these signs, outdoor furniture that generateselectricity or visual light effects, transparent, semi-transparent, oropaque roadside sound barriers that generate electricity or visuallighting effects, etc.

In a specific embodiment, extraterrestrial applications are included:‘solar sails’ for spacecraft propulsion may be equipped with the LSCaccording to the invention, for simultaneously generation of electricityvia PV cells irradiated by the LSC collected radiation (and optionallyalso direct irradiation by the sun) as well as propelling the craft.

Some specific embodiments of a solar cell and the LSC according to theinvention are depicted in FIGS. 14 a-14 e. These figures are similar toFIG. 1 described above. Of course other configurations are possible,like for instance those depicted in FIGS. 2,3,4,5 a, 5 b and 16 a-16 c.These schematic drawings are only shown to illustrate possibleconstructions:

-   -   a. A solar cell 13 placed sideways, such that light from        waveguide 2 is concentrated into solar cell or photovoltaic cell        13.    -   b. A solar cell 13 placed at the “bottom”, for instance        intercepting optional mirror layer 9, such that light from        waveguide 2 is concentrated into solar cell or photovoltaic cell        13.    -   c. A solar cell 13 placed at the “top”, for instance        intercepting optional cholesteric layers 7 and 8, such that        light from waveguide 2 is concentrated into solar cell or        photovoltaic cell 13.    -   d. A solar cell 13 placed at the “top”, for instance        intercepting optional cholesteric layers 7 and 8 and luminescent        layer 1, such that light from waveguide 2 is concentrated into        solar cell or photovoltaic cell 13.    -   e. A solar cell 13 placed at the “bottom”, for instance        intercepting optional mirror layer 9 and waveguide 2, such that        light from waveguide 2 is concentrated into solar cell or        photovoltaic cell 13.

FIGS. 14 a-14 e also show the optional alignment layer 20, as describedabove (for instance a PI layer). Such alignment layers are known to theperson skilled in the art.

Another embodiment is schematically shown in FIG. 15. Herein a number(especially two or more) of solar concentrators comprising theluminescent layer 1 and waveguide 2 are used to provide solar light tosolar cell 13. For instance, the LSC can be as depicted in FIGS. 1-8.Light from the waveguide 2 may be transported to solar cell 13 viawaveguides (fibres) 26. Optionally, the light concentrated in waveguide2 may be collimated by collimators 25 into waveguides 26.

As will be clear to the person skilled in the art, the schematicembodiments of FIGS. 14 a-14 e do also include embodiments wherein anumber of solar cells 13 are incorporated in the combination ofluminescent layer 1 and waveguide 2. For instance, in case luminescentlayer 1 and waveguide 2 (and optional other layers, as described above)are in the form of a flat or substantial flat laminate, at least part ofone or more of the edges of the waveguide 2 laminate may be opticallycoupled to a number of solar cells or PV cells 13. Hence, in anembodiment there is provided a window comprising the luminescent objectaccording to the invention and a photovoltaic cell (or more than one, aswill be clear to the person skilled in the art) capable of convertingoptical radiation to electrical energy which is optically coupled to theluminescent object.

Further, one LSC comprising a luminescent layer 1 and waveguide 2according to the invention may be coupled to more than 1 fibre 26 andmay thus provide light to more than one PV cells 13.

In yet another embodiment, the LSC comprising the luminescent layer 1and waveguide 2 being essentially a thin sheet of plastic film, like atransparency slide, with or without an adhesive backing. The film wouldcontain aligned or unaligned dye molecules, and a preferablyselectively-reflective layer (for instance this may be cholestericlayers 9 a and/or 9 b). This film could then be mounted by the end-useron any window. Pre-placed within the window frame will be thephotovoltaic. Thus, the window will become the waveguide 2, transportingthe light to the solar cell(s) 13 in the frame. The film may bedisposable: that is, it may be peeled off the window to allow naturalsunlight back into the room, if desired.

In an embodiment, the terms “optically coupled” or “coupled optically”also include the optical coupling of objects of which surfaces are notadjacent, but between which a distance can be present. For instance, thephotovoltaic cell 13 is preferably adjacent to waveguide 2, but in anembodiment, there may be some space in between. Such space may forinstance be filled with air or comprise a vacuum, or even polymer.Polymer may for instance be used for attaching a PV cell to thewaveguide, as in an embodiment a sheet of high-index thermosettingplastic may be applied to aide in extraction of light from the waveguideand introduction of this light into the PV. For example, putting alow-refractive index polymer between the metallic mirror on the bottomand the dye layer or waveguide seems to increase the output of thesystem.

The invention is also directed to embodiments wherein the luminescentobject comprises next to an aligned polymeric layer (with alignedphotoluminescent material) further comprises one or more layerscomprising unoriented photoluminescent material.

In an embodiment, the luminescent object further comprises at least onewavelength selective and polarisation selective reflecting cholestericlayer (shown as layers 7 or 8). The luminescent object may also comprise(a stack of) two or more wavelength selective and polarisation selectivereflecting cholesteric layers. For instance, the luminescent object maycomprise a right and left handed cholesteric layer, but may alsocomprise two right handed cholesteric layers, or a stack of right, rightand left or right, left right handed cholesteric layers as wavelengthselective layers. Such an embodiment is schematically indicated in FIG.16 c, with a wavelength selective mirror comprising instead of a righthanded and a left handed cholesteric layers 7 and 8, respectively,comprises for instance a stack of three layers 8 (being either allleft-handed or all right-handed (see also FIG. 3)). As will be clear tothe person skilled in the art, this embodiment may also be applicable toother embodiments, as for instance schematically shown in the otherfigures. To give an example, also the fibre of FIGS. 7 and 8 maycomprise instead of layers 7 and 8, comprise one or more cholestericlayer(s) reflecting right-handed circularly polarized light or one ormore cholesteric layer(s) reflecting left-handed circularly polarizedlight or comprises both one or more cholesteric layer(s) reflectingright-handed circularly polarized light and one or more cholestericlayer(s) reflecting left-handed circularly polarized light.

As mentioned above, the position of luminescent layer 1 and waveguide 2may be exchanged (see for instance also FIGS. 7 and 8), which isschematically shown in FIGS. 16 a and 16 b, which are variants on FIGS.1 and 14 b, but may also be applied as variants to other embodiments.

In the case that only one cholesteric layer reflecting right-handed orleft-handed circularly polarized light is applied, either as top layer7(or 8) as for instance shown in FIG. 13 a (see also the embodiments ofFIGS. 1-5 b; 14 a-e; 16 a-b), or as bottom layer 9 (see the embodimentof FIG. 2) or both such top 7 (or 8) and bottom layers (9), thereflectivity to emission radiation may be smaller than at least 50%. Atleast 25% may suffice, or at least 50% of light with the specificpolarization (right-handed or left-handed circularly polarized,respectively). Hence, in a specific embodiment, a luminescent object isprovided comprising:

a. a luminescent layer or core containing a photoluminescent material;and

b. a wavelength-selective mirror;

wherein the luminescent layer or luminescent core is optically coupledto the wavelength-selective mirror, said wavelength-selective mirrorbeing at least 50% transparent to light absorbed by the photoluminescentmaterial and at least 25% reflective to radiation that is emitted by thephotoluminescent material and wherein the wavelength-selective mirrorcomprises a cholesteric layer of chiral nematic polymer.

Especially, a luminescent object is provided, wherein the polymericwavelength-selective mirror comprises one or more cholesteric layer(s)reflecting right-handed circularly polarized light or one or morecholesteric layer(s) reflecting left-handed circularly polarized lightor comprises both one or more cholesteric layer(s) reflectingright-handed circularly polarized light and one or more cholestericlayer(s) reflecting left-handed circularly polarized light. The specificembodiments further described in the claims, especially claims 3, 7,11-27 are also applicable to this embodiment

In yet a further embodiment, instead of or in addition to downconversion photoluminescent material (i.e. material that absorbs lightat a shorter wavelength than it emits light, for instance a green to redconverter), the luminescent object of the invention may also compriseupconversion materials (i.e. material that absorbs light at a shorterwavelength than it emits light).

The invention is further illustrated by means of the following examples.

EXAMPLES Example 1

A clean glass substrate has a polyimide alignment layer (Optimer Al1051, ex JSR Micro) spin cast on it at 2000 rpm/s at an acceleration of2500 rpm/s² for 45 sec. After this the substrate was heated for 1.5hours at 180° C. under vacuum. The alignment layer was rubbed with avelvet cloth to induce a planar alignment of the applied cholestericliquid crystal.

To the opposite side of the slide, an isotropic dye-doped mixture wasapplied. The mixture was prepared by mixing 1 wt. % Irgacure 184 (exCiba Chemicals) and 1 wt. % DCM(4-Dicyanmethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (exAldrich Chemicals) together with a solution containing 75 wt. %dipentaerythritol pentaacrylate (Aldrich) and 25 w % ethanol. A roughly10 μm thick film was produced by spin coating at 4000 rpm for 45 seconds(2500 rpm/sec ramp speed). The sample was UV cured (λ=365 nm) under a N₂atmosphere for 10 minutes at room temperature. The absorption andemission spectra of the sample may be seen in the FIG. 9 as the boldsolid and dotted curves, respectively.

A 120 nm silver mirror was sputtered on top of the dye layer using aconventional sputter coater (Emitech K575X sputter coater, at a currentof 65 mA for 2.5 min).

A cholesteric mixture was made by mixing 3.9 wt. % of a right-handedchiral dopant LC756 (ex BASF), 1 wt. % Irgacure 184 (ex Ciba Chemicals),1 wt. % surfactant and 94 wt. % reactive liquid crystal host RM257(Merck) dissolved in xylene (55 wt. % solids, 45 wt. % solvent).Subsequently the mixture was stirred at 80° C. for 3 hours.

The cholesteric mixture was spin cast on the polyimide alignment layerat 2000 rpm for 45 sec. with an acceleration speed of 2500 rpm/s. Afterspin casting the substrate was heated at 80° C. for about 1 min. toevaporate the solvent and allow the surfactant to improve the alignmentof the chiral nematic liquid crystal. Afterwards, photopolymerizationwas carried out at room temperature by irradiation for 10 min using aUV-lamp (peak wavelength 365 nm) in a N₂-environment. A second,left-handed cholesteric could then be easily applied to the surfaceusing a similar process. Finally, a layer of reflective silver paint wasapplied to three edges of the waveguide, resulting in the final device.

The above procedure was repeated two times using 4.2 and 4.5 wt. % ofthe right-handed chiral dopant LC756. FIG. 9 depicts the transmissionspectra of the cholesteric layer containing 3.9 wt. % of theright-handed chiral dopant exposed to unpolarized light at three anglesof incidence, i.e. 0°, 20° and 40°. The transmission spectra for thecholesteric layers containing 4.2 wt. % or 4.5 wt. % of the chiraldopant are essentially identical except that the reflection band for thelayers containing 4.2 wt. % and 4.5 wt. % occur at significantly shorterwavelengths. To be more precise, the reflection band for normal lightincidence in the 4.2 wt. % sample is blue-shifted by about 32 nm and thereflection band for the 4.5 wt. % sample by about 83 nm.

The light output of the LSC samples was determined using an Autronic DMS703 (Melchers GmbH) together with a CCD-Spect-2 (CCD-Camera). The LSCsamples were placed in a custom-made sample holder and exposed to aroughly uniform light source located at a distance of about 11 cm. Lightoutput from a small area (about 0.8 cm²) of the emission edge of thesample was measured through an angular distribution of −50-50° in stepsof 1 degree. The total emission was determined by integrating thespectra over all measured wavelengths (350-800 nm) and all angles.Measurements along the length of the sample edge indicate littlevariation with position, but for these experiments emission measurementposition was fixed. All measurements for single samples with multiplelayers were taken on the same day. A sketch of the measurement setup isshown in FIG. 10.

Amount of chiral Angle of incidence Sample dopant used 0° 20° 40° A 3.9wt. % 17%   14%  11% B 4.2 wt. % 4% 0.6%  −8% C 4.5 wt. % 1%  −6% −14%

As may be clearly seen, the application of the single cholesteric layer(Sample A) improved light output at least 11-17%, and by application ofa second, left-handed layer this improvement will be on the order of20-35%. The results obtained for samples B and C are reduced because thereflection band of the cholesteric layer partly coincides with theabsorption peak of the photoluminescent material and/or only partlycoincides with the emission peak of said photoluminescent material.

Herein, the alignment layer was used to align the cholesteric layers.

This experiment was repeated, but using a pmma substrate in place of aglass substrate. A polyimide alignment layer (Optimer A1 1051, ex JSRMicro) was spin cast on it at 2000 rpm/s at an acceleration of 2500rpm/s² for 45sec. After this the substrate was heated for 1.5 hours at100° C. in air. The alignment layer was rubbed with a velvet cloth toinduce a planar alignment of the applied cholesteric liquid crystal.

To the opposite side of the slide, an isotropic dye-doped mixture wasapplied. The mixture was prepared by mixing 1 wt. % Irgacure 184 (exCiba Chemicals) and 1 wt. % DCM(4-Dicyanmethylene-2-methyl-6-(p-dimethylaminostyryl)-4H -pyran (exAldrich Chemicals) together with a solution containing 75 wt. %dipentaerythritol pentaacrylate (Aldrich) and 25 w % ethanol. A roughly10 μm thick film was produced by spin coating at 4000 rpm for 45 seconds(2500 rpm/sec ramp speed). The sample was UV cured (λ=365 nm) under a N₂atmosphere for 10 minutes at room temperature.

A 120 nm silver mirror was sputtered on top of the dye layer using aconventional sputter coater (Emitech K575X sputter coater, at a currentof 65 mA for 2.5 min).

A cholesteric mixture was made by mixing 4.1 wt. % of a right-handedchiral dopant LC756 (ex BASF), 1 wt. % Irgacure 184 (ex Ciba Chemicals),1 wt. % surfactant and 94 wt. % reactive liquid crystal host RM257(Merck) dissolved in xylene (55 wt. % solids, 45 wt. % solvent).Subsequently the mixture was stirred at 80° C. for 3 hours.

The cholesteric mixture was spin cast on the polyimide alignment layerat 2000 rpm for 45 sec. with an acceleration speed of 2500 rpm/s. Afterspin casting the substrate was heated at 80° C. for about 1 min. toevaporate the solvent and allow the surfactant to improve the alignmentof the chiral nematic liquid crystal. Afterwards, photopolymerizationwas carried out at room temperature by irradiation for 10 min using aUV-lamp (peak wavelength 365 nm) in a N₂-environment. A second,left-handed cholesteric could then be easily applied to the surfaceusing a similar process. Finally, a layer of reflective silver paint wasapplied to three edges of the waveguide, resulting in the final device.

The light output of the LSC sample was determined using an Autronic DMS703 (Melchers GmbH) together with a CCD-Spect-2 (CCD-Camera). The LSCsamples were placed in a custom-made sample holder and exposed to aroughly uniform light source located at a distance of about 11 cm. Lightoutput from a small area (about 0.8 cm) of the emission edge of thesample was measured through an angular distribution of −70-70° in stepsof 1 degree. The total emission was determined by integrating thespectra over all measured wavelengths (350-800 nm) and all angles. Forthis experiment emission measurement position was fixed.

The sample with the single, right-handed cholesteric demonstrated a 34%increase in integrated light output for input light normal to thewaveguide surface when compared to the integrated light output of thebare dye layer. When subsequently covered with the second, left-handedcholesteric the total integrated light output was determined to be 53%greater than from the dye layer alone for input light incident normal tothe waveguide surface.

Example 2

A homeotropic dye-doped liquid crystal mixture was applied to a clean 30mm×30 mm×1 mm glass slide. The liquid crystal mixture was prepared bymixing an ethanol solution which contained 1 wt. % Irgacure 184 (ex CibaChemicals) and 1 wt. % Coumarin 30 (ex Aldrich Chemicals) together witha solution containing 50 wt. % RMM77 monomer and a 50 w % xylene in aweight ratio of 1:1. RMM77 (Merck) is a nematic homeotropic reactiveliquid crystal from which the main components are the liquid crystalsRM82 and RM257 (both Merck) and a homeotropic dopant. The mixture wasstirred at 80° C. for 2 hours until all ethanol was evaporated. Thexylene was evaporated by applying the mixture on the preheatedwaveguides (80° C.) for 10 minutes. After evaporation of the xylene awet film was created with a 24 μm Meyer rod, resulting in anapproximately 10 μm thick film. The samples were UV cured (λ=365 nm)under a N₂ atmosphere for 4 minutes at room temperature and then for 4minutes at 110° C.

The measurement of light output by the sample was carried out by anAutronic DMS 703 (Melchers GmbH) together with a CCD-Spect-2(CCD-Camera). The LSC samples were placed in a custom-made sample holderand exposed to a collimated light source. Light output from the surfaceof the sample was measured through an angular distribution of 0-90° insteps of 1 degree. The peak emission was determined and compared to thepeak emission from the surface of an isotropic sample prepared in theexact manner as described above, but using a RM82 and RM257 mixture notcontaining the homeotropic dopant. The result was the homeotropic samplealmost halved the amount of light lost through the sample, therebyincreasing the amount of light directed into the waveguide. FIG. 11depicts the result of this experiment. In this figure, light intensityemitted from the surface is given as a function of emission angle fortwo samples, one with an isotropically aligned dye (circles) and onewith the homeotropically aligned dye (triangles). Note that for thewaveguide used in this experiment, all light above 33° was trapped bytotal internal reflection, and thus could not escape through thesurface. The waveguide herein comprises the glass plate, havingdimensions of 30×30×1 mm (l×w×h).

The performance of this LSC is improved substantially by the applicationof a right-handed and left-handed cholesteric layer coupled with thesilver mirrors as described in Example 1.

Example 3

Example 2 is repeated except that a liquid crystalline polymer isemployed that is aligned at a tilt angle of around 30° using theprocedure described by Sinha et al in Appl. Phys. Lett. (2001), 79 (16),2543-2545.

Again the efficiency of the LSC is measured using the methodologydescribed in Example 1. The results show that the efficiency of the LSCexceeds the efficiency of the LSCs described in examples 1 and 2. Thissuperior efficiency is believed to be associated with an improvedincoupling of the emitted radiation into the waveguide.

Example 4

A tilted alignment of the dye molecules was also achieved in thefollowing manner. A solution was made of a two polyimides: 4% NissanG1211, a homeotropic polyimide, and 96% Nissan G130, a planar polyimide.This solution was spin cast on a 30×30×1 mm glass plate at 5000 rpm for60 seconds, and cured in a vacuum oven for 90 minutes at 180° C. Thepolyimide was rubbed on a velvet cloth.

A mixture containing 1% of a perylene-based dye, 1% of thephotoinitiator Irg184 (Ciba chemicals) and 98% RM257 (Merck) dissolvedin a 55:45 wt % ratio in xylene was spin cast on the polyimide alignmentlayers at 2000 rpm for 40 seconds, and placed on a hot plate at 80° toevaporate the solvent. This procedure resulted in a tilt-angle of theperylene-based dyes of about 15°.

The light output of the LSC sample was determined using an Autronic DMS703 (Melchers GmbH) together with a CCD-Spect-2 (CCD-Camera). The LSCsamples were placed in a custom-made sample holder and exposed to aroughly uniform light source located at a distance of about 11 cm. Lightoutput from a small area (about 0.8 cm²) of the emission edge of thesample was measured through an angular distribution of −70-70° in stepsof 1 degree. The total emission was determined by integrating thespectra over all measured wavelengths (350-800 nm) and all angles. Forthis experiment emission measurement position was fixed. The totalintegrated light output of this sample was 10% higher than the output ofa randomly oriented sample prepared in the same manner (which did notcomprise a polyimide alignment layer).

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. In the claims, any reference signsplaced between parentheses shall not be construed as limiting the claim.Use of the verb “to comprise” and its conjugations does not exclude thepresence of elements or steps other than those stated in a claim. Thearticle “a” or “an” preceding an element does not exclude the presenceof a plurality of such elements. The invention may be implemented bymeans of hardware comprising several distinct elements, and by means ofa suitably programmed computer. In the device claim enumerating severalmeans, several of these means may be embodied by one and the same itemof hardware. The mere fact that certain measures are recited in mutuallydifferent dependent claims does not indicate that a combination of thesemeasures cannot be used to advantage.

The invention claimed is:
 1. A luminescent object comprising: aluminescent layer or core containing a photoluminescent material and awavelength-selective mirror; wherein the luminescent layer orluminescent core is optically coupled to the wavelength-selectivemirror; wherein the wavelength-selective minor is at least 50%transparent to light absorbed by the photoluminescent material and atleast 50% reflective to radiation that is emitted by thephotoluminescent material; wherein the wavelength-selective minorcomprises a cholesteric layer of chiral nematic polymer, the cholestericlayer comprises one;

and wherein the reflectivity of the wavelength-selective mirror forradiation emitted by the photoluminescent material exceeds thereflectivity of the same minor for optical radiation absorbed by thephotoluminescent material by at least 50%.
 2. The luminescent objectaccording to claim 1, wherein the cholesteric layer of thewavelength-selective mirror comprises a first cholesteric layerreflecting right-handed circularly polarized light and a secondcholesteric layer reflecting left-handed circularly polarized light. 3.The luminescent object according to claim 1, wherein thewavelength-selective mirror further comprises a first polymeric stacklayer reflecting one polarization of light and a second polymeric stacklayer reflecting an opposite polarization of light.
 4. The luminescentobject according to claim 1, wherein the reflectivity of thewavelength-selective mirror for radiation emitted by thephotoluminescent material exceeds the reflectivity of the same minor foroptical radiation absorbed by the photoluminescent material by at least80%.
 5. The luminescent object according to claim 1, wherein thewavelength-selective mirror is at least 60% reflective to radiation thatis emitted by the photoluminescent material.
 6. The luminescent objectaccording to claim 1, wherein the wavelength-selective mirror is atleast 70% reflective to radiation that is emitted by thephotoluminescent material.
 7. The luminescent object according to claim1, wherein the wavelength-selective mirror further comprises a mirrorwhich is at least 50% transparent to light absorbed by thephotoluminescent material and that is at least 50% reflective topolarized radiation.
 8. The luminescent object according to claim 1,wherein the object is predominantly transparent for optical radiation inthe range of 400-700 nm.
 9. The luminescent object according to claim 1further comprising a luminescent layer and a waveguide, wherein theluminescent object is an optical laminate or an optical fiber; whereinthe luminescent object is coupled optically to the waveguide, theluminescent object comprises an aligned polymer that contains anoriented photoluminescent material; wherein the orientedphotoluminescent material is immobilized within the aligned polymer; andwherein the aligned polymer has a pretilt angle of 10-90° relative tothe surface of the object.
 10. The luminescent object according to claim1, wherein the object is an optical fiber comprising a luminescent layerand a waveguide core.
 11. The luminescent object according to claim 10,wherein the luminescent layer or luminescent core and thewavelength-selective mirror are adjacent.
 12. The luminescent objectaccording to claim 1 wherein the wavelength-selective mirror is at least60% transparent to light absorbed by the photoluminescent material. 13.The luminescent object according to claim 12, wherein thewavelength-selective mirror is at least 70% transparent to lightabsorbed by the photoluminescent material.
 14. The luminescent objectaccording to claim 1, wherein the luminescent layer or luminescent corecomprises an aligned polymer that contains an oriented photoluminescentmaterial.
 15. The luminescent object according to claim 14, wherein theobject is an optical fiber comprising a luminescent core and awaveguide.
 16. The luminescent object according to claim 14, wherein thewaveguide does not comprise a fluorescent dye.
 17. The luminescentobject according to claim 14, wherein the pretilt angle is within therange of 15°-85°.
 18. The luminescent object according to claim 14,wherein the pretilt angle is within the range of 30°-60°.
 19. Theluminescent object according to claim 14, wherein the orientedphotoluminescent material has a dichroic ratio of at least 2.0 in aplanar cell.
 20. The luminescent object according to claim 14, whereinthe oriented photoluminescent material has a dichroic ratio of at least3.0 in a planar cell.
 21. The luminescent object according to claim 14,wherein the oriented photoluminescent material has a dichroic ratio ofat least 5.0 in a planar cell.
 22. The luminescent object according toclaim 14, wherein the aligned polymer has a pretilt angle of 30°-80°.23. The luminescent object according to claim 14, wherein the alignedpolymer has a pretilt angle of 30°-70°.
 24. The luminescent objectaccording to claim 14, wherein the aligned polymer has a pretilt angleof 40°-70°.
 25. The luminescent object according to claim 14, whereinthe aligned polymer has a pretilt angle of 40°-60°.
 26. A photovoltaicdevice comprising: a photovoltaic cell and an electromagnetic radiationcollection medium; wherein the photovoltaic cell is capable ofconverting optical radiation to electrical energy and is opticallycoupled to a luminescent object; wherein the electromagnetic radiationcollection medium comprises the luminescent object; the luminescentobject comprises: a luminescent layer or core containing aphotoluminescent material and a wavelength-selective mirror; wherein theluminescent layer or luminescent core is optically coupled to thewavelength- selective mirror; wherein the wavelength-selective minor isat least 50% transparent to light absorbed by the photoluminescentmaterial and at least 50% reflective to radiation that is emitted by thephotoluminescent material; wherein the wavelength-selective minorcomprises a cholesteric layer of chiral nematic polymer, the cholestericlayer comprises one of

and wherein the reflectivity of the wavelength-selective mirror forradiation emitted by the photoluminescent material exceeds thereflectivity of the same mirror for optical radiation absorbed by thephotoluminescent material by at least 50%.
 27. A window comprising aluminescent object and a photovoltaic cell; wherein the photovoltaiccell is capable of converting optical radiation to electrical energy andis optically coupled to the luminescent object; wherein the luminescentobject comprises a luminescent layer or core containing aphotoluminescent material and a wavelength-selective mirror; wherein theluminescent layer or luminescent core is optically coupled to thewavelength-selective mirror; wherein the wavelength-selective minor isat least 50% transparent to light absorbed by the photoluminescentmaterial and at least 50% reflective to radiation that is emitted by thephotoluminescent material; wherein the wavelength-selective minorcomprises a cholesteric layer of chiral nematic polymer, the cholestericlayer comprising one of

and wherein the reflectivity of the wavelength-selective mirror forradiation emitted by the photoluminescent material exceeds thereflectivity of the same mirror for optical radiation absorbed by thephotoluminescent material by at least 50%.
 28. A method of concentratingincident optical radiation comprising subjecting the incident opticalradiation to a luminescent object comprising: a luminescent layer orcore containing a photoluminescent material and a wavelength-selectiveminor; wherein the luminescent layer or luminescent core is opticallycoupled to the wavelength- selective mirror; wherein thewavelength-selective minor is at least 50% transparent to light absorbedby the photoluminescent material and at least 50% reflective toradiation that is emitted by the photoluminescent material; wherein thewavelength-selective minor comprises a cholesteric layer of chiralnematic polymer, the cholesteric layer comprising one of

wherein the reflectivity of the wavelength- selective minor forradiation emitted by the photoluminescent material exceeds thereflectivity of the same mirror for optical radiation absorbed by thephotoluminescent material by at least 50%.